REPETITIVE HYDRAULIC FRACTURING AND SHEAR ZONE INFLATION IN AN ARCHEAN GRANITOID-HOSTED, RIBBON BANDED, AU-QUARTZ VEIN SYSTEM, RENABIE AREA, ONTARIO, CANADA

Economically significant tonalite(similar to 2722 Ma)- and trondhjemit e(similar to 2728 Ma)-hosted Archean Au-quartz vein mineralization in the Renabie area, Ontario, located in a marginal part of the 10-15 km thick Wawa Domal Gneiss Terrane, shows strong structural control. Majo r ore zones occur in similar to E/W and similar to NW-NNW striking duc tile-brittle high strain zones that are superimposed on an earlier reg ional foliation. Mineralized zones are up to similar to 20-30 m wide, mineable to a depth of similar to 1300 vertical metres, and dip typica lly at 60 degrees-80 degrees angles. Production and reserves for the R enabie mine totalled similar to 6 million tomes at 6.6 g/t Au and 2 g/ t Ag. Net shear, from kinematic indicators, is oblique reverse; south side-up, sinistral on - E/W structures (e.g., Nudulama; Renabie main z one, depth > 2475 level) and west side-up, sinistral on similar to NW- NNW structures (e.g,, Braminco #21 zone). The strain features of the s hear zone-hosted mineralization, together with evidence of reaction-en hanced ductility associated with hydrothermal fluid/wallrock interacti on, indicate that mineralization and hydrothermal alteration occurred synchronously with shear deformation. The characteristic 'banded' appe arance (book/ribbon structure) of the Renabie veins, which is typical of many Archean and Cordilleran Au-quartz vein systems, is interpreted to be the result of inflationary vein formation by repeated increment s of hydraulic fracturing and mineral deposition (similar to 250-750) associated with hydrothermal fluid overpressuring within and below the actively deforming ductile-brittle shear zones (crack-seal type proce ss). Fracture orientation was largely controlled by the anisotropy in rock tensile strength associated with the approximately planar shear f abric. The element/mineral association of Renabie is typical of oxidiz ed Archean Au-quartz vein systems containing anhydrite, hematite, carb onate and tourmaline with a chalcophile association consisting of MoS2 , Cu/Pb sulphides, native Au, Ag and Pb-Bi-Au-Ag tellurides. Key facto rs in the development of significant mineralization at Renabie, as com pared with other usually small, low grade, Archean granitoid-hosted Au -quartz vein systems, include the repetitive nature of the mineralizin g process, and the focusing of Au-bearing fluids within the actively d eforming shear zones. A specific and sufficiently large reservoir of o verpressured hydrothermal Archean Au-bearing fluid was generated at so me location, not yet identified, in the Archean crustal structure repr esented by the Kapuskasing section. (C) 1998 Elsevier Science B.V. All rights reserved.